Method for preparing iron phosphate by ammonium method with controllable morphology

Abstract

The invention discloses a shape-controllable method for preparing iron phosphate by an ammonium method, and relates to the technical field of preparation of battery-grade iron phosphate. Weighing ferrous sulfate heptahydrate to prepare a saturated solution, placing the saturated solution in an ultrasonic reactor, and starting ultrasonic treatment; preheating to a set temperature, sequentially adding a battery-grade phosphate solution, an oxidizing agent and a dispersing agent, adjusting the pH value of a reaction system to be acidic, and carrying out heat preservation reaction after charging is completed; filtering and washing after the reaction is completed, taking a filter cake, mixing the filter cake, aging for crystal transformation until the color is changed, filtering and washing the filter cake, drying, calcining and crushing to obtain a battery-grade iron phosphate product. According to the method, the surface tension among the iron phosphate particles is reduced, direct contact and agglomeration among the particles are prevented by forming an effective adsorption layer on the surfaces of the particles, the formation of secondary particles is effectively controlled, and the problem of secondary particle agglomeration commonly existing in the iron phosphate synthesis process is effectively solved; and the uniformity and dispersity of the iron phosphate product are greatly improved.

Description

Technical Field

[0001] The present invention relates to the technical field of preparing battery-grade iron phosphate, and particularly relates to a method for preparing iron phosphate by an ammonium method with controllable morphology. Background Art

[0002] As a kind of cathode material for lithium-ion batteries, lithium iron phosphate shows broad application prospects in fields such as electric vehicles, energy storage systems, and portable electronic devices due to its high theoretical capacity, good thermal stability, low cost, and environmental friendliness. Iron phosphate, as the precursor of lithium iron phosphate, has also received extensive attention. The market demand has increased, and at the same time, higher requirements have been put forward for the performance of iron phosphate, mainly focusing on enhancing the thermal stability of the material to facilitate the subsequent lithium iron phosphate material to obtain higher energy density, better cycle stability, and faster charge and discharge efficiency.

[0003] Lithium iron phosphate has an orthorhombic olivine structure, pnma space group. Its crystal structure is composed of FeO6 octahedrons and PO4 tetrahedrons, forming a stable three-dimensional structure through specific connection methods. In LiFePO4, lithium atoms are inserted into the layered structure of iron phosphate to form LiFePO4 units. Iron phosphate and lithium iron phosphate have extremely high similarity in crystal structure and unit cell parameters. Both show similar layered or olivine structures, which makes it very easy for iron phosphate to be converted into lithium iron phosphate through a lithiation reaction during the preparation of lithium iron phosphate without undergoing a large-scale structural reorganization.

[0004] Since the material structure and volume before and after lithium deintercalation / insertion are almost unchanged, and the volume difference between the two is only 6.8%, during the process of preparing lithium iron phosphate using iron phosphate as the raw material, the fine regulation of the crystal structure and morphology of iron phosphate can optimize the specific surface area, pore structure, and active site distribution of the material, thereby affecting the insertion / extraction rate and efficiency of lithium ions, and having a great impact on the physical and chemical indexes and electrochemical performance of the lithium iron phosphate product. Therefore, the improvement of the performance of iron phosphate mainly depends on the regulation of its morphology.

[0005] The existing morphologies of iron phosphate include spherical, rod-shaped, and oval-shaped. The morphology regulation mainly conducts research based on the existing sodium method:

[0006] CN202110384792.7 discloses a method for preparing iron phosphate with controllable morphology, using ferrous sulfate as the iron source and sodium hydrogen phosphate solution as the phosphorus source. By changing the amount of monosodium hydrogen phosphate / disodium hydrogen phosphate and phosphoric acid added in the existing sodium method, the regulation of the microscopic morphology of iron phosphate is realized, and spherical-like and petal-shaped iron phosphate are obtained.

[0007] A method for preparing flaky iron phosphate is disclosed in CN201910659570.4. Phosphoric acid and a crystal conversion agent containing citrate are added to a ferrous ion solution to complex with ferric ions, reduce the formation rate of iron phosphate precipitation, obtain primary iron phosphate particles with larger particle sizes, and have complete crystal growth. In addition, after the citrate in the crystal conversion agent complexes with ferric ions, it can selectively adsorb on different crystal planes and induce radial crystal growth. The primary particles change from rice grain shape to flaky shape, and a flaky iron phosphate product is obtained.

[0008] A method for preparing flaky iron phosphate by a sodium process and the flaky iron phosphate prepared thereby are disclosed in CN202010063212.X. Lamellar iron phosphate can be prepared by adding morphology control agents such as hexamethylenediamine and terephthalic acid. The two ligand sites of the control agent can induce the growth direction of the crystal, cause the generated iron phosphate to aggregate and grow in the horizontal direction, form flaky primary particles, and then the primary particles gradually agglomerate into petal-shaped secondary iron phosphate particles.

[0009] The above prior arts all conduct research on the morphology of iron phosphate based on the sodium process, and the above regulation methods cannot be applied to the morphology regulation of iron phosphate in the ammonium process.

[0010] In the existing methods for synthesizing lithium iron phosphate, such as the method disclosed in CN202410160133.9, a dispersant is first mixed with a ferrous sulfate solution, then a carbon source is added and burned and oxidized to achieve carbon doping, and then a lithium source and a phosphorus source are added and spray-dried to obtain particles with a particle size of 1.5 μm. After curve temperature-controlled calcination, carbon-coated lithium iron phosphate is obtained. This method is a one-step method, which cannot effectively solve the problem of secondary particle agglomeration commonly existing in the synthesis process of iron phosphate, cannot regulate the morphology of the obtained iron phosphate product, and even more cannot obtain an iron phosphate product with good processing performance. Summary of the Invention

[0011] The purpose of the present invention is to provide a method for preparing iron phosphate by an ammonium process with controllable morphology, so as to solve the problems that the existing methods are difficult to be applicable to the preparation of iron phosphate by the ammonium process or have poor effects.

[0012] To solve the above technical problems, the present invention adopts the following technical solutions: A method for preparing iron phosphate by an ammonium process with controllable morphology, which is characterized by including the following steps:

[0013] S1. Weigh ferrous sulfate heptahydrate and prepare it into a saturated solution, place it in an ultrasonic reactor and turn on the ultrasonic wave.

[0014] S2. After preheating to the set temperature, sequentially add a battery-grade phosphate solution, an oxidant, and a dispersant, and adjust the pH of the reaction system to acidic. After the feeding is completed, keep the reaction at a constant temperature.

[0015] S3. After the reaction is completed, filter and wash. After taking the filter cake and preparing a slurry, age and transform the crystal until it changes color, then filter, wash the filter cake, and dry, calcine, and crush it to obtain a battery-grade iron phosphate product.

[0016] A further technical solution is that the set temperature in S2 is 35 - 50 °C.

[0017] A further technical solution is that the battery-grade phosphate solution in S2 is any one of ammonium monohydrogen phosphate and ammonium dihydrogen phosphate, the oxidant is hydrogen peroxide, and the dispersant is sodium dodecylbenzenesulfonate or polyvinylpyrrolidone.

[0018] A further technical solution is that the phosphorus content in the battery-grade phosphate solution is 4% - 12%, the iron content in the saturated solution of ferrous sulfate heptahydrate is 3% - 7%, the concentration of hydrogen peroxide is 20% - 30%, the dosage of the dispersant is 0.5 - 3‰ of the mass of the reaction solution, and the molar ratio of ferrous sulfate heptahydrate, battery-grade phosphate, and oxidant is 1:1:0.5 - 0.7.

[0019] A further technical solution is that the specific steps for adjusting the pH of the reaction system are to use ammonia water to adjust the pH value to 1.0 - 3.0.

[0020] A further technical solution is that the heat preservation reaction time in S2 is 25 - 35 min.

[0021] A further technical solution is that in S3, the washing is carried out with pure water multiple times until the conductivity of the washing water <

[0022] 3.5 ms / cm.

[0023] A further technical solution is that the solid content after preparing the slurry in S3 is 6% - 20%.

[0024] A further technical solution is that the aging and crystal transformation in S3 are carried out at a temperature of 90 - 95 °C for 1 - 4 h.

[0025] A further technical solution is that the drying in S3 is carried out at 100 °C for 2 - 5 h, the calcination temperature is 550 - 650 °C, and the calcination time is 2 - 8 h.

[0026] Compared with the prior art, the beneficial effects of the present invention are:

[0027] 1) The method provided by this application is applicable to the preparation of iron phosphate by the ammonium method. In this method, ferrous sulfate heptahydrate is first prepared into a saturated solution and preheated. While undergoing ultrasonic treatment, phosphate, an oxidant, and a dispersant are added, and the pH is adjusted. After heat preservation reaction, the filter cake is washed, slurried, aged, and crystal-transformed. By the above method, the surface tension between iron phosphate particles is reduced. By forming an effective adsorption layer on the particle surface, direct contact and agglomeration between particles are prevented, the formation of secondary particles is effectively controlled, and the problem of secondary particle agglomeration commonly existing in the synthesis process of iron phosphate is effectively solved, greatly improving the uniformity and dispersibility of the iron phosphate product.

[0028] 2) A dispersant is used in this application, which can significantly improve the fluidity of the reaction system, facilitate the full mixing and contact between reactants, promote the uniform progress of the reaction, and cooperate with the ultrasonic process to bring a certain amount of energy input, generating a microscopic stirring effect. The cavitation effect, micro-jet flow, and acoustic pressure fluctuation generated by ultrasonic waves in the reaction solution can produce intense local turbulence and strong microscopic stirring, significantly enhancing the mass transfer and collision frequency between reactant molecules. This non-contact energy transfer method not only avoids the impurity introduction and shear force damage that may be brought by traditional mechanical stirring but also promotes the uniform mixing of reactants at the molecular scale, ensuring the homogeneous nucleation and growth of iron phosphate crystals.

[0029] 3) When the iron phosphate obtained in this application is used as a precursor to prepare lithium iron phosphate, its superiority is fully demonstrated. Due to the excellent morphology uniformity, moderate particle size, and good dispersibility of the precursor iron phosphate, the prepared lithium iron phosphate not only retains a large specific surface area, providing more active sites for the carbothermal reduction reaction and accelerating the reaction rate but also effectively prevents particle re-agglomeration during subsequent processing. This structural characteristic not only improves the ion diffusion efficiency and electron transport ability of lithium iron phosphate but also enhances its structural stability during charge and discharge processes, thus significantly improving the electrochemical performance of the battery material. After the iron phosphate obtained by this method is further prepared into lithium iron phosphate, the first charge of the obtained lithium iron phosphate at 0.1C is 162.23 mAh·g -1 ; the first discharge at 0.1C; 161.21 mAh·g -1 ; the first efficiency at 0.1C is 99.4%, showing good electrochemical performance. Description of the Drawings

[0030] Figure 1 SEM images of the morphology of the iron phosphate obtained in Example 1; among them, a) is a picture of the iron phosphate prepared in Example 1 magnified 50,000 times; b) is a picture of the iron phosphate prepared in Example 1 magnified 5,000 times;

[0031] Figure 2SEM micrograph of the morphology of the iron phosphate obtained in Comparative Example 1; among them, c) is a picture of the iron phosphate prepared in Comparative Example 1 magnified 45,000 times; d) is a picture of the iron phosphate prepared in Comparative Example 1 magnified 5,000 times. Detailed implementation mode

[0032] In order to make the objectives, technical solutions and advantages of the present invention clearer, the following further details the invention in conjunction with the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0033] Example 1: Preparation of battery-grade iron phosphate product

[0034] Prepare a solution of battery-grade monoammonium phosphate (P mass concentration is 10%, monoammonium phosphate purity ≥ 99%). Take ferrous sulfate heptahydrate to prepare a saturated solution (iron mass concentration is 6%), and the concentration of hydrogen peroxide is 20%.

[0035] Place the obtained saturated solution of ferrous sulfate heptahydrate in an ultrasonic reactor and preheat it to 45°C. Turn on the ultrasound, the ultrasound frequency is 40 kHz, and sequentially add the monoammonium phosphate solution, hydrogen peroxide, and sodium dodecylbenzenesulfonate. The molar ratio of ferrous sulfate, monoammonium phosphate, and hydrogen peroxide is 1:1:0.55, and the addition amount of sodium dodecylbenzenesulfonate is 1‰ of the mass of the reaction solution. Then add ammonia water to adjust the pH of the reaction system to 2.0.

[0036] After the feeding is completed, keep the reaction at a constant temperature for 30 minutes, filter, wash the filter cake with pure water until the conductivity of the washing water < 3.5 ms / cm. After the washing is completed, adjust the filter cake to a slurry with a solid content of 8%. Heat the slurry in a glass reaction kettle to 92°C and age for 2 hours. Filter the filter cake and wash it with pure water until the conductivity of the washing water < 2.5 ms / cm. After the washing is completed, dry it at 100°C for 3 hours, and then calcine it at 600°C for 5 hours. After crushing, a battery-grade iron phosphate product is obtained. The SEM morphology diagram of the obtained product is as Figure 1 shown.

[0037] Comparative Example 2: Preparation of battery-grade iron phosphate product, adding ultrasound during the aging process

[0038] Prepare a solution of battery-grade monoammonium phosphate (P mass concentration is 10%, monoammonium phosphate purity ≥ 99%). Take ferrous sulfate heptahydrate to prepare a saturated solution (iron mass concentration is 6%), and the concentration of hydrogen peroxide is 20%.

[0039] Place the obtained saturated solution of ferrous sulfate heptahydrate in an ultrasonic reactor and preheat it to 45°C. Turn on the ultrasonic wave with a frequency of 40 kHz, and sequentially add the ammonium dihydrogen phosphate solution, hydrogen peroxide, and sodium dodecylbenzenesulfonate. The molar ratio of ferrous sulfate, ammonium dihydrogen phosphate, and hydrogen peroxide is 1:1:0.55, and the addition amount of sodium dodecylbenzenesulfonate is 1‰ of the mass of the reaction solution. Then add ammonia water to adjust the pH of the reaction system to 2.0.

[0040] After the feeding is completed, keep the reaction at a constant temperature for 30 min, filter, and wash the filter cake with pure water until the conductivity of the washing water is < 3.5 ms / cm. After the washing is completed, adjust the filter cake to a solid content of 8%. Heat the slurry to 92°C in the ultrasonic reactor and age for 2 h. Filter the filter cake and wash it with pure water until the conductivity of the washing water is < 2.5 ms / cm. After the washing is completed, dry it at 100°C for 3 h, then calcine it at 600°C for 5 h, and crush it to obtain a battery-grade iron phosphate product.

[0041] Example 3: Preparation of battery-grade iron phosphate product

[0042] Prepare a solution of battery-grade ammonium dihydrogen phosphate (P mass concentration is 10%, ammonium dihydrogen phosphate purity ≥ 99%). Take ferrous sulfate heptahydrate to prepare a saturated solution (iron mass concentration is 6%), and the hydrogen peroxide concentration is 20%.

[0043] Place the obtained saturated solution of ferrous sulfate heptahydrate in an ultrasonic reactor and preheat it to 45°C. Turn on the ultrasonic wave with a frequency of 40 kHz, and sequentially add the ammonium dihydrogen phosphate solution, hydrogen peroxide, and polyvinylpyrrolidone. The molar ratio of ferrous sulfate, ammonium dihydrogen phosphate, and hydrogen peroxide is 1:1:0.55, and the addition amount of polyvinylpyrrolidone is 1‰ of the mass of the reaction solution. Then add ammonia water to adjust the pH of the reaction system to 2.0.

[0044] After the feeding is completed, keep the reaction at a constant temperature for 30 min, filter, and wash the filter cake with pure water until the conductivity of the washing water is < 3.5 ms / cm. After the washing is completed, adjust the filter cake to a solid content of 8%. Heat the slurry to 92°C in a glass reaction kettle and age for 2 h. Filter the filter cake and wash it with pure water until the conductivity of the washing water is < 2.5 ms / cm. After the washing is completed, dry it at 100°C for 3 h, then calcine it at 600°C for 5 h, and crush it to obtain a battery-grade iron phosphate product.

[0045] Polyvinylpyrrolidone has a certain volatility, which has an impact on human health and the environment, and will corrode pipelines and equipment after entering the iron phosphate synthesis system, and it is difficult to handle later.

[0046] Example 4: Preparation of battery-grade iron phosphate product

[0047] Prepare a solution of monoammonium phosphate at the battery grade (P mass concentration is 10%, and the purity of monoammonium phosphate is ≥99%). Prepare a saturated solution of ferrous sulfate heptahydrate (iron mass concentration is 6%), and the concentration of hydrogen peroxide is 20%.

[0048] Place the obtained saturated solution of ferrous sulfate heptahydrate in an ultrasonic reactor and preheat it to 45°C. Turn on the ultrasound with a frequency of 40 kHz, and sequentially add the monoammonium phosphate solution, hydrogen peroxide, and sodium dodecylbenzenesulfonate. The molar ratio of ferrous sulfate, monoammonium phosphate, and hydrogen peroxide is 1:1:0.55, and the addition amount of sodium dodecylbenzenesulfonate is 1‰ of the mass of the reaction solution. Then add ammonia water to adjust the pH of the reaction system to 1.3.

[0049] After the feeding is completed, keep the reaction at a constant temperature for 30 minutes, filter, and wash the filter cake with pure water until the conductivity of the washing water is <3.5 ms / cm. After the washing is completed, adjust the filter cake to a slurry with a solid content of 8%. Heat the slurry in a glass reaction kettle to 92°C and age for 2 hours. Filter the filter cake and wash it with pure water until the conductivity of the washing water is <2.5 ms / cm. After the washing is completed, dry it at 100°C for 3 hours, then calcine it at 600°C for 5 hours, and obtain the battery-grade iron phosphate product after crushing.

[0050] Comparative Example 5:

[0051] Prepare a solution of monoammonium phosphate at the battery grade (P mass concentration is 10%, and the purity of monoammonium phosphate is ≥99%). Prepare a saturated solution of ferrous sulfate heptahydrate (iron mass concentration is 6%), and the concentration of hydrogen peroxide is 20%.

[0052] Place the obtained saturated solution of ferrous sulfate heptahydrate in a glass reaction kettle and preheat it to 45°C, and sequentially add the monoammonium phosphate solution and hydrogen peroxide. The molar ratio of ferrous sulfate, monoammonium phosphate, and hydrogen peroxide is 1:1:0.55.

[0053] After the feeding is completed, keep the reaction at a constant temperature for 30 minutes, filter, and wash the filter cake with pure water until the conductivity of the washing water is <3.5 ms / cm. After the washing is completed, adjust the filter cake to a slurry with a solid content of 8%. Heat the slurry in a glass reaction kettle to 92°C and age for 2 hours. Filter the filter cake and wash it with pure water until the conductivity of the washing water is <2.5 ms / cm. After the washing is completed, dry it at 100°C for 3 hours, then calcine it at 600°C for 5 hours, and obtain the battery-grade iron phosphate product after crushing. The SEM morphology diagram of the obtained product is as Figure 2 shown.

[0054] Perform performance tests on the iron phosphate products obtained in Examples 1-4 and the comparative examples according to the existing detection methods, and the obtained results are shown in Table 1 below.

[0055] Table 1: Main index table of iron phosphate prepared in each example and comparative example

[0056]

[0057]

[0058] The iron phosphate products obtained in Examples 1-4 and Comparative Example 1 were respectively mixed with glucose, lithium carbonate and additives, and then prepared into lithium iron phosphate through ball milling, spray drying, calcination and grinding. The performance of the obtained lithium iron phosphate was detected according to the existing detection methods, and the results are shown in Table 2.

[0059] Table 2: Main index table of lithium iron phosphate prepared from iron phosphate in each example and comparative example

[0060]

[0061] There were differences in the ultrasonic process, dispersant type, and pH value of iron phosphate synthesis in Examples 1-4. As can be seen from Tables 1-2, when sodium dodecylbenzenesulfonate was used as the dispersant and ultrasonic-assisted treatment was utilized during the precipitation synthesis stage, the obtained iron phosphate had a moderate specific surface area and tap density, high crystallinity, and uniform particle size distribution. The introduction of ultrasonic technology had a certain cavitation effect in the reaction solution, forming local turbulence and micro-stirring, promoting the uniform mixing of reactants at the molecular scale, and ensuring the homogeneous nucleation and growth of iron phosphate crystals. Therefore, the primary particle size of the iron phosphate products obtained in each example was small, with good uniformity and dispersibility, and there was no obvious agglomeration phenomenon.

[0062] As can be seen from Table 2, the 1C cycle retention rate of lithium iron phosphate obtained from the iron phosphate by the method provided in this application was good, and the highest could reach 98.8%.

[0063] Using the iron phosphate product with small primary particles and good dispersibility obtained in Example 1 to prepare lithium iron phosphate was helpful to improve the structural stability of lithium iron phosphate, reduce the volume expansion and contraction effects during charge and discharge, reduce the shedding of active materials, and thus improve the cycle life and have a high capacity retention rate. At the same time, the high-energy disturbance of ultrasonic waves would adjust the crystal structure of iron phosphate, and after preparing iron lithium, it would promote the repair of defects inside the crystal, optimize the crystal orientation, making the transmission paths of electrons and ions smoother, thereby further improving the electrochemical performance. See specifically Figure 1 a)-b).

[0064] Comparative Example 5 used traditional reaction technology to prepare iron phosphate. For the morphology of the obtained iron phosphate material, see specifically Figure 2c) - d). Due to the lack of sufficient dispersion mechanism and effective control of inter - particle repulsive force during the reaction process, it is difficult to form a uniform dispersion system among particles. There are serious adhesion and agglomeration phenomena between primary particles. Secondly, due to the uneven contact and accumulation between particles, the uniformity of the product is severely affected, with a wide particle size distribution and various morphologies. This hinders the electron and ion transport paths in the subsequently prepared lithium iron phosphate, reducing the conductivity and ion diffusion rate of the material, and ultimately affecting the electrochemical performance of the battery.

[0065] In Comparative Example 2, ultrasonic technology was introduced during both the reaction precipitation and aging stages. However, while reducing the particle size of the iron phosphate product, it also caused the problem of decreased crystallinity. This shows that excessive ultrasonic assistance leads to too high surface energy of crystal nuclei, inhibiting the agglomeration and recrystallization processes between crystal nuclei, and thus hindering the growth of crystals in the direction of larger size and higher crystallinity. As a result, the crystal growth of the product is incomplete, with poor crystallinity, directly affecting the physical and chemical properties of the iron phosphate product and its performance in battery applications.

[0066] Example 6

[0067] Prepare a solution of battery - grade monoammonium phosphate (P mass concentration is 8%, purity of monoammonium phosphate ≥ 99%). Prepare a saturated solution of ferrous sulfate heptahydrate (iron mass concentration is 7%), and the concentration of hydrogen peroxide is 25%.

[0068] Place the obtained saturated solution of ferrous sulfate heptahydrate in an ultrasonic reactor and preheat it to 40 °C. Turn on the ultrasonic wave with a frequency of 40 kHz, and sequentially add the monoammonium phosphate solution, hydrogen peroxide, and sodium dodecylbenzenesulfonate. The molar ratio of ferrous sulfate, monoammonium phosphate, and hydrogen peroxide is 1:1:0.6, and the addition amount of sodium dodecylbenzenesulfonate is 1.5‰ of the mass of the reaction solution. Then add ammonia water to adjust the pH of the reaction system to 2.5.

[0069] After feeding, keep the reaction at a constant temperature for 35 min, filter, wash the filter cake with pure water until the conductivity of the washing water < 3.5 ms / cm. After washing, adjust the filter cake to a solid content of 12%. Heat the slurry in a glass reaction kettle to 95 °C and age for 2.5 h. Filter the filter cake and wash it with pure water until the conductivity of the washing water < 2.5 ms / cm. After washing, dry it at 100 °C for 3 h, then calcine it at 600 °C for 5 h, and obtain the battery - grade iron phosphate product after crushing.

[0070] Example 7

[0071] Prepare a solution of battery - grade monoammonium phosphate (P mass concentration is 12%, purity of monoammonium phosphate ≥ 99%). Prepare a saturated solution of ferrous sulfate heptahydrate (iron mass concentration is 6.5%), and the concentration of hydrogen peroxide is 20%.

[0072] Place the obtained saturated solution of ferrous sulfate heptahydrate in an ultrasonic reactor and preheat it to 45 °C. Turn on the ultrasound with an ultrasonic frequency of 40 kHz, and sequentially add the ammonium monophosphate solution, hydrogen peroxide, and sodium dodecylbenzenesulfonate. The molar ratio of ferrous sulfate, ammonium monophosphate, and hydrogen peroxide is 1:1:0.5, and the addition amount of sodium dodecylbenzenesulfonate is 2‰ of the mass of the reaction solution. Then add ammonia water to adjust the pH of the reaction system to 1.8.

[0073] After the feeding is completed, keep the reaction at a constant temperature for 30 min, filter, and wash the filter cake with pure water until the conductivity of the washing water is < 3.5 ms / cm. After the washing is completed, adjust the filter cake to a slurry with a solid content of 15%. Heat the slurry in a glass reaction kettle to 90 °C and age for 3.5 h. Filter the filter cake and wash it with pure water until the conductivity of the washing water is < 2.5 ms / cm. After the washing is completed, dry it at 100 °C for 3 h, then calcine it at 600 °C for 5 h, and crush it to obtain a battery-grade iron phosphate product.

[0074] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing iron phosphate by the ammonium process with controllable morphology, characterized in that The steps include: S1. Weigh ferrous sulfate heptahydrate to prepare a saturated solution, place it in an ultrasonic reactor and turn on ultrasound; S2. After preheating to the set temperature, add battery-grade phosphate solution, oxidant, dispersant in sequence, and adjust the pH of the reaction system to acidic. After the addition is completed, keep the reaction warm; S3. After the reaction is completed, the filter cake is filtered and washed, and the filter cake is taken for slurrying and then aged and crystallized until it changes color. The filter cake is filtered and washed, and then dried, calcined and crushed to obtain a battery-grade iron phosphate product.

2. The method according to claim 1, characterized in that: The temperature in S2 is set to 35-50°C.

3. The method according to claim 1, characterized in that: The battery-grade phosphate solution in S2 is any one of ammonium dihydrogen phosphate and ammonium monohydrogen phosphate, the oxidant is hydrogen peroxide, and the dispersant is sodium dodecylbenzene sulfonate or polyvinyl pyrrolidone.

4. The method according to claim 2, characterized in that: The phosphorus content of the battery-grade phosphate solution is 4% to 12%, the iron content of the saturated ferrous sulfate heptahydrate solution is 3% to 7%, the concentration of hydrogen peroxide is 20% to 30%, the amount of dispersant used is 0.5 to 3‰ of the mass of the reaction solution, and the molar ratio of ferrous sulfate heptahydrate, battery-grade phosphate, and oxidant is 1:1:0.5 to 0.

7.

5. The method according to claim 1, characterized in that: The specific step of adjusting the pH of the reaction system is to use ammonia water to adjust the pH value to 1.0-3.

0.

6. The method according to claim 1, characterized in that: The heat preservation reaction time in S2 is 25 to 35 minutes.

7. The method according to claim 1, characterized in that: The washing in S3 is performed by using pure water for multiple washings until the conductivity of the washing water is less than 3.5 ms / cm.

8. The method according to claim 1, characterized in that: The solid content of S3 after slurry adjustment is 6% to 20%.

9. The method according to claim 1, characterized in that: The aging and crystallization in S3 is carried out at a temperature of 90 to 95° C. for 1 to 4 hours.

10. The method according to claim 1, characterized in that: The drying in S3 is carried out at 100° C. for 2 to 5 hours, the calcination temperature is 550 to 650° C., and the calcination time is 2 to 8 hours.

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